As an example of how I started DIY, and still use this method frequently.
Flavor #1
Pick a main flavor like a Fruit.
TFA Blueberry Wild 5% (OR) FE Blackberry 2% (notice the difference in %'s)(some are stronger so beware)
Sometimes Flavor #2 & #3 could be other fruits...
Flavor #2
Decide if you want a Custard, Cheesecake, Yogurt type, Vanilla & Cream, etc.
A. CAP Vanilla Custard v1 1-5% (OR) TFA Vanilla Custard 3-10% (amounts can vary to taste here, strong or mild)
OR
B. CAP NY Cheesecake 1-5% (OR) TFA Cheesecake Graham 2-5% (mostly the same as Custard types)
OR
C. CAP Creamy Yogurt 5% (or) CAP Greek Yogurt (5%) (Greek is more tart than Creamy, other brands work fine too)
OR
D. TFA Vanilla Swirl 3% + TFA/CAP Sweet Cream 1-2% (for a Vanilla Cream type)
E. TFA/CAP Vanilla Bean Ice Cream, TFA French Vanilla Deluxe, Hangsen Cream can work well too.
Flavor #3
TFA Brown Sugar .5% - 3% (add a more sugary baked flavor)
FA Coconut 0.5% (can add a creamy buttery effect to some mixes)
FA Nut Mix, FA Nut Mix, FA Walnut, FW Hazelnut (used in small amounts to add nutty in bakery vapes usually)
TFA 5% AP (used in small amounts for nutty effects also)
TFA/CAP Sweet Cream 1%, any Marshmallow 1-3%, Whipped Cream, FA Meringue, Bavarian Cream 1-3% (sweetness and/or body)
FA Oba Oba, FA Liquid Amber, Cinnamon, FA Cardamom, etc.
Some of the flavors from #3 can be moved down to Flavor # 4, 5, 6, etc.
And some flavors like FW Butter Pecan at 10%, or FW Cinnamon Roll 7%, work great as single flavor vapes.
Many others are fine as single flavors too.
Or even a 2 flavor mix like FA Pomegranate 2.5% + Hangsen Cream 0.5%
Also study the recipes section to get more ideas, experimentation can be fun and rewarding.